(a) Field of the Invention
The present invention relates to an IC test system for testing IC packages such as BGA (ball grid array) packages.
(b) Description of the Related Art
BGA LSIs are increasingly used along with a higher integration of the LSI packages. FIGS. 1A and 1B are side view and a bottom view of a package of a BGA LSI. The BGA package 11 is substantially of a square having a 5xc3x975 mm to 50xc3x9750 mm size with a thickness of 0.5 to 3 mm. A large number of solder balls 12 are mounted on the bottom surface of the package at the peripheral area thereof, and a LSI 13 is embedded at the central area of the package. A plurality of bonding wires extend within the BGA package between the LSI 13 and the solder balls 12. The BOA package is mounted on a printed circuit board with the bottom surface mounting thereon the solder balls 12 being in contact with the top surface of a printed circuit board, and coupled thereto by thermally melting the solder balls for electric and mechanical connection between the solder balls and the terminals of the printed circuit board.
The solder balls 12 are arranged in an array on the outer periphery of the package. Each of the solder balls 12 is of a sphere having a diameter of around 0.3 mm and projects from the bottom surface. The number of the solder balls 12 in a LSI package may be as high as several hundreds to a thousand in a high-integrated LSI, wherein the pitch of the solder balls may be as low as 0.5 mm. In view of the higher integration of the next generation LSI together with demand for a smaller package size on the printed circuit board, a higher density for the arrangement of the solder balls 12 is desired in the BGA package.
For testing or measuring the electric characteristics of the BGA package, a test instrument (or test socket) is generally used which has an array of contact pins arranged in a pitch corresponding to the pitch of the solder balls 12. The contact pins are attached to the test socket so that the contact pins advance and retract to/from the socket by a spring function. More specifically, the contact pin has a compression spring at the bottom thereof and the contraction of the spring provides a thrust force for a suitable electric contact between the contact pin and the solder ball. Thus, a tip of the contact pin exerts a thrust force to the solder ball during the abutment thereto.
FIGS. 2A to 2D show sectional views of a depression of a test socket, consecutively show the advancement of a BGA package 11 toward the contact pins 15 disposed in the depression. A suction head 16 holds the BOA package 11 at the top surface thereof by vacuum suction and moves in the horizontal direction for positioning of the BGA package 11 with respect to the test socket 14. The suction tool 16 then moves downward or toward the depression 17 of the socket 14, as shown in FIG. 2A, and allows the BGA package 11 to drop within the depression 17 by stopping the vacuum suction, as shown in FIG. 2A. The depression 17 has a tapered wall portion at each side of the square and an additional straight wall portion at the bottom of the depression 17.
The BGA package 11 slides along the tapered wall of the depression 17, as shown in FIG. 2B, and stops at the straight wall portion, as shown in FIG. 2C, where the solder balls 12 are aligned with the contact pins 15 of the test socket 14. The suction head 16 then further moves downward to thrust the BGA package 11 with a specific contact pressure, as shown in FIG. 2D, to obtain electric contacts between the solder balls 12 and the contact pins 15. Thereafter, an electric test is conducted for the BGA package 11,
FIG. 3 shows the structure of a conventional IC test system, which includes a base 10, a carry-in tray 20, a first carriage arm 21, a positioning member 22, a second carriage arm 23, socket 14, and a carry-out tray 24. Both the carriage arms 21 and 23 have respective suction heads 16 for holding and releasing the BGA package 11. In the drawing, a top cover is not shown which covers the top surface or table of the base 10 of the test system.
The first carriage arm 21 moves straight, then allows the suction head 16 to move in the vertical direction, thereby transferring the BGA packages 11 before testing one by one from the carry-in tray 20 to the positioning member 22, which positions the BOA package 11 based on the external shape thereof.
The second carriage arm 23 turns horizontally about its vertical axis to allow the suction head 16 to move in the horizontal direction, thereby carrying the positioned BGA package 11 to the socket 14. The BGA package 11 is dropped therein, as described before, and subjected to testing thereof. After the testing, the BGA package 11 is transferred to the carry-out tray 24 and temporarily stored thereon for waiting carry-out thereof.
In the conventional IC test system, as shown in FIG. 4A, it sometimes happens that the BGA package 11 is not smoothly dropped by the suction head 16 to the bottom of the depression along the taper wall, whereby an edge of the BGA package 11 is stopped by the taper wall. In this case, the suction head 16 which thrusts the BGA package 11 against the contact pins 15 may damage the BGA package 11 by an excessive thrust force exerted from above. This often happens when the second carriage arm 23 drops the BGA package 11 before the vibration of the second carriage arm 23 is stopped, or when the socket 14 receives a vibration (or waggle) from other members.
The excessive external force is also applied to the contact pins 15 in a horizontal direction as well as the vertical direction. The contact pins 15 are extendable in the vertical direction to some extent as described before. However, the contact pins 15 are susceptible to the horizontal thrust force and are bent thereby due to the fragility of the contact pins 15 having a diameter around 0.3 mm. Even if only one of the contact pins 15 is bent and cannot move in the vertical direction, the testing cannot be not carried out because the all the contact pins 15 are not likely in electrical contact with the solder balls 12.
In addition, it may also happen that the contact pins 15 are not in a suitable abutment against the solder balls even if the BGA package 11 is dropped to the bottom of the depression 17. This happens when the solder balls 12 are not correctly arranged with respect to the external shape of the BGA package 11. For example, if the orientation of the solder ball array is deviated from the orientation of the BGA package 11, the contact pins are not in a suitable abutment against the centers of the solder balls and are thus subjected to an undesirable external force in the horizontal direction upon the thrust by the suction head 16. This may cause peeling-off of the solder balls 12 from the BGA package 11 or damage to the contact pins 15.
In either case, a failed contact pin must be replaced by a new contact pin. The contact pin is fragile due to the extremely small diameter thereof and is difficult to settle on the socket without a skill. This necessitates the replacement of the whole socket on the user side of the test system, the cost of which is a heavy burden on the user.
The present inventors proposed in an earlier Japanese application No. 11-230237 for solving the above problems, wherein the positioning of the BGA package is carried out on the basis of the arrangement of the respective solder balls, the image of which is taken by a CCD camera and stored beforehand. This allows the solder balls to be in a suitable alignment with the contact pins In addition, the proposed technique uses a suction head which thrusts the BGA package so as to allow the solder balls to be in abutment against the contact pins while holding the BGA package without dropping.
The proposed technique achieved a significant solution for the above problems; however, it was not in fact satisfactory. This is because the vibration of the test system causes a failure in the electric contact between the solder balls and the contact pins.
More specifically, in the proposed test system, the main components such as the carriage arms, test socket and positioning member are respectively fixed onto the top table of the test system by using bolts and nuts. Among others, the carriage arms cause a vibration (or waggle) on the test system, which is not necessarily transferred to the socket. That is, it happens that the vibration is left on the carriage arm whereas the socket is not subjected to the vibration or waggle. If a significant time length is inserted for waiting the convergence of the vibration on the carriage arm, a satisfactory test result may be obtained. However, in this case, the throughput of the testing is degraded.
On the other hand, there may be a case wherein the test socket is subjected to vibration of other members such as a tray drive when the carriage arm moves downward to thrust the BGA package toward the socket without vibration of the carriage arm itself. The socket is likely subjected to a vibration because the vicinity of the socket is hardly provided with a reinforcement member for preventing the vibration of the test system partly due to the presence of a cable box used for the socket. In this case, a correct test is not conducted either.
In particular, in the case of a higher integrated BGA package such as mounting thereon solder balls having a diameter of 0.3 mm and arranged with a pitch of 0.5 mm, a correct test is difficult to achieve due to small vibrations or waggles respectively occurring on the socket and associated members Along with the higher integration and increased number of solder balls in the BGA packages, the above problems are considered to be critical in the test of the BGA packages.
It is an object of the present invention to provide an improved test system, which is capable of solving the above problems caused by the vibrations or waggles in the test system.
The present invention provides a IC test system comprising a base a socket including a plurality of contact pins, a positioning member for positioning an IC package with respect to the contact pins, a carriage arm for carrying the IC package positioned by the positioning member to the socket so that external terminals of the IC package are in contact with is the contact pins, and an IC tester for testing electric characteristics of the IC package, wherein the socket, the positioning member and the carriage arm are fixed to a common plate member fixed to the base.
In accordance with the present invention, the common plate member transfers the vibration caused by one of the socket, positioning member and the carriage arm in the test system equally to the other of the socket, positioning member and the carriage arm, whereby these members vibrate similarly to each other and thus an incorrect positioning caused by the vibration as encountered in the conventional test system can be prevented.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.